This invention relates to the preparation of high purity inorganic materials, and, more particularly, to the preparation of compounds of sulfur, selenium and tellurium having low contaminant content.
Compounds of sulfur, selenium and tellurium with cations are sometimes termed chalcogenides. A number of such compounds have important technological applications, but their use is inhibited by the inability to prepare the compounds with very low impurity levels, and in particular with low oxygen and oxide contents.
One such compound, arsenic triselenide, having a chemical formula of As.sub.2 Se.sub.3, is a material having high transparency to light in the infrared range. Of commonly available materials, only KCl has greater transparency to light having a wavelength of 10.6 micrometers. Arsenic triselenide has sufficiently different chemical properties from KCl that it is of interest in a number of infrared applications for use in displays and windows, as well as other uses where KCl cannot be used.
The use of arsenic triselenide has, in the past, been inhibited by the unavailability of this material in a highly purified form, free of oxygen and oxide contamination. Even moderate levels of oxygen or oxide, on the order of about 100 parts per millions or more, can be detrimental to the infrared transparency of arsenic triselenide.
The preparation of arsenic-containing compounds must be done very carefully, as many compounds of arsenic are highly toxic. Arsenic triselenide is conventionally prepared by a sublimation process from the elements. Arsenic sublimes upon heating at atmospheric pressure. In a closed vitreous silica system, arsenic vapor is sublimed from the solid element, transferred via the vapor phase to another chamber containing solid selenium, and reacted with the selenium. It is known that the solid elemental arsenic, from which the vapor is sublimed, contains arsenic oxide in an amount that increases from the center of the arsenic mass to the surface. After the sublimation is complete, a fine residue of arsenic oxide in the form of a web is observed, leading to the belief that the oxide is not transported from the solid arsenic to the compound through the vapor phase, to contaminate the compound. The process thus relies upon the assumption of a distillation-like effect for purification of oxide from the arsenic prior to reaction with the selenium.
Unfortunately, the conclusion that oxide is not transported through the vapor phase during the sublimation process is incorrect. Arsenic triselenide obtained by this process contains a substantial amount of oxygen impurity, typically on the order of above 100 parts per million oxygen. This level of oxygen impairs the infrared transparency of the compound, and makes it technically noncompetitive in most applications.
There therefore exists a need for an improved process for preparing high purity compounds of sulfur, selenium and tellurium. Although the need is most acute for, and has been discussed here in relation to, arsenic triselenide, the use of other compounds of this family are similarly hampered by the unavailability of the high-purity material, free of oxygen, oxide, and other contaminants. Such a process should result in a very low oxygen content for the compound. The present invention fulfills this need, and further provides related advantages.